Theoretical Study of the Antioxidant Activity of Vitamin E:  Reactions of α-Tocopherol with the Hydroperoxy Radical

The reactivity of the hydroperoxy radical with α-tocopherol [Formula: see text] a prototype of the chemical reactions involved in biological antioxidant actions [Formula: see text] was studied theoretically. Two pathways were analyzed: hydrogen abstraction from the phenolic hydrogen and hydroperoxy addition to the aromatic ring. The reaction paths for the two mechanisms were traced independently, and the respective thermal rate constants were calculated using variational transition-state theory with multidimensional small-curvature tunneling. The reactivity of the hydroperoxy radical was found to be dominated by the hydrogen abstraction mechanism on α-tocopherol, with a rate constant of 1.5 × 10(5) M(-1) s(-1) at 298 K. It was also found that the mechanism of the reaction is not direct but passes through two intermediates, one of which may have a significant role in preventing the pro-oxidant effects of α-tocopherol.